1. Field of the Invention
This invention relates generally to the fabrication of a MR sensor. In particular it relates to an MR sensor in which a free layer is biased by a layer of high coercivity FePt-containing material.
2. Description of the Related Art
With the ever increasing areal density with which data is stored on magnetic media, such as magnetic disks in hard disk drives (HDD), the magneto-resistive (MR) sensor that is used as the read-back element in the HDD is required to have a correspondingly improved spatial resolution while achieving and maintaining a reasonable signal-to-noise ratio (SNR). Referring to schematic FIGS. 1a, 1b and 1c, there are shown three views of a generic, prior-art current-perpendicular-to-plane (CPP) MR read head. FIG. 1a illustrates the read head in a vertical cross-sectional plane parallel to its air bearing surface (ABS) plane. FIG. 1b illustrates the read head from an overhead view of a horizontal cross-sectional plane through its magnetically free layer (discussed below). FIG. 1c is a portion of the illustration of FIG. 1a, isolating the sensor stack portion of the head.
Referring to FIG. 1a, there is shown the MR head, which could be a CPP-GMR head (current perpendicular-to-plane giant-magneto-resistive head), in which there is a current that passes perpendicularly to the active magnetic layers, through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the giant-magneto-resistive effect. Alternatively, the head could be a CPP-TMR (current perpendicular-to-plane tunneling magneto-resistive) head, in which there is a current that passes perpendicularly to the active magnetic layers through the entire head structure and in which the resistance of the head varies in accord with the physical principles of the tunneling-magneto-resistive effect. Either of these particular types of head, which are state-of-the-art read-back heads, will be improved by the advantageous properties of the hard biasing layer of the present invention.
FIG. 1a shows the following physical elements of the generic prior art head. Looking vertically downward, there is first shown an upper (or top) shield (1) that protects the sensor stack (6) from extraneous magnetic fields. At the bottom of the head, there is shown a substrate, that is typically a corresponding lower (or bottom) shield (2) that performs a similar task at the bottom edge of the sensor. Thus the sensor is protected by a pair of shields at some desired separation (3).
Hard bias (HB) magnets (4) (magnets formed of high coercivity magnetic material) are laterally disposed to either side of the sensor stack (6). These magnets, which stabilize the magnetization of the free layer (8) are positioned between the shields (1), (2) and their magnetizations are shown as arrow (5). These hard magnetic layers, which are the subject of this invention, are formed on seedlayers (20) that promote the requisite crystalline anisotropy. The sensor stack itself (6) is typically formed as a patterned lamination of five horizontal layers, formed beneath an upper capping layer (18). An arrow (7) shows the direction of magnetization of the magnetically free layer of the sensor stack, as seen in FIG. 1c. 
FIG. 1b is a horizontal cross-sectional slice through the two HB layers (4) and the magnetically free layer (8) of the sensor stack, as will be discussed below.
Referring to FIG. 1c, there is shown a schematic, illustration of the isolated sensor stack (6) of FIG. 1a showing the following five horizontal layers: the magnetically free layer (8), showing its magnetization vector as an arrow (7); a layer (9) that is a dielectric layer that serves as a tunneling barrier layer for the TMR sensor, or is a conducting layer for the GMR type sensor, a reference layer (10) relative to whose magnetization the free layer magnetization moves, a coupling layer (eg. a layer of Ru) (11), a pinned layer (12) whose magnetization is held spatially fixed by a thick layer (19) of antiferromagnetic material that also pins layer (10). The hard biasing layers (4), with longitudinal magnetization (5), provides a biasing magnetic field in the sensor stack (6) to orient the magnetization (7) of the free layer (8) in a longitudinal direction. A capping layer (18) is positioned between the free layer (8) and the upper shield (1). In forming the sensor, the stack and the hard bias layers are defined by a single etching process that insures they are at the same height.
Recently much attention has been given to improving the hard biasing of the MR sensor. The state-of-the-art magnetic biasing element is usually a structure comprising a Cr containing seedlayer, followed by a CoCrPt or CoPt hard magnetic thin film. The magnetic biasing field provided by the bias film must be sufficiently high to achieve stabilization of the free layer. As sensors are made smaller and smaller to achieve the required high areal recording density, as discussed above, the free layer becomes more volatile magnetically and more difficult to bias.
Much interest has been shown in L10 FePt film as a hard magnetic substance due to its higher saturation magnetization and high coercivity. Qiu et al., (US Publ Pat. Appl. 2009/0274931) discloses 20Pt/Fe62Pt38/20Pt film that achieves a coercivity of 5100 Oe after being annealed for 4 hrs at 300° C. Hua et al. (US Publ. Pat. Appl. 2010/0047627) disclosed a 10Pt/10Fe40Pt60/150 A Fe65Pt35/10Fe40Pt60/10Pt film that achieved a remanent coercivity of about 6000 Oe after being annealed for 4 hrs at 300° C. In both disclosures, a Pt capping layer and seedlayer are key to achieving high coercivities at moderate annealing conditions.
The cited prior art above, as well as other prior art relating to the use of various seedlayers and capping layers in forming magnetic films of high coercivity under moderate annealing condition, has led us to consider some particularly advantageous combinations that are particularly suitable for the hard biasing of MR sensors. Other prior art relating to this area includes Mao et al. (US Publ. Pat. Appl. 2002/0015268), Larson et al. (U.S. Pat. No. 7,061,731) and Sbiaa et al. (US Publ. Pat. Appl. 2006/0114620). Although this prior art discloses the use of FePt films together with seedlayers and capping layers, the combinations described herein and their advantageous properties are not disclosed.